The present patent application claims the priority benefit of the filing date of European Application (EPO) No. 98202916.7 filed Sep. 1, 1998
The invention generally relates to a data communication system, and more specifically to a system and method for decrypting an encrypted message and a method for broadcasting data.
Such systems are known in various embodiments and are for example used in a decoder with a conditional access module for pay TV. Generally a secret key is required for decrypting the message, wherein decryption is carried out in a security device in order to prevent unauthorized persons to access the secret key. As security device a smart card can be used, for example. Such a known system using a smart card for decrypting the message shows the disadvantage that the security device has a restricted computing capacity. On the other hand on many locations a computer system with high computing capacity is available, however such systems are easily accessible for unauthorized persons.
The invention aims to provide a system of the above-mentioned type with a combination of high security and high computing capacity.
To this end the system for decrypting an encrypted message according to the invention comprises first and second decryption devices, the first decryption device having a higher security than the second decryption device, means for dividing the encrypted message into blocks, and means for providing at least the first block of the message to the first decryption device and for providing a plurality of the further blocks of this message to the second decryption device, wherein an output of the first decryption device is used as input of the second decryption device, said second decryption device operating according to a block chaining method for decrypting said plurality of further blocks.
In this manner a system is provided wherein the first decryption device having a higher security is used for decrypting a first block of the message only whereafter the remaining part of the message is decrypted by the second decryption device which can have a high computing capacity. The second decryption device can have a low security as the use of a block chaining method makes the insecure decryption device as secure as the first decryption device.
In order to further enhance security the providing means provides each xth block to the first decryption device according to a further embodiment of the invention. It is noted that the term xth block means that the number of intermediate blocks is not fixed, i.e. may very as desired.
The invention further provides a method for decrypting an encrypted message, comprising the steps of dividing a message into blocks, decrypting at least the first block in a first decryption device, decrypting a plurality of further blocks in a second decryption device, the first decryption device having a higher security than the second decryption device, using an output of the first decryption device as input of the second decryption device and operating the second decryption device according to a block chaining method.